I held off on upgrading Photoshop CS3 to CS4 earlier because (a) it grates on me to spend more money buying a new version when they haven't fixed the bugs in the old version yet, (b) because they were asking close to $1000 for an upgrade, never mind full version, (c) because I didn't see any compelling reason in the new version, mostly just incremental changes, and (d) because of some alarming reports about the performance of the new version. (Especially from Darksome.)
CS4 has been out for quite a while now and I can buy the CS3 -> CS4 upgrade for under $300, which is a bit more reasonable. Doubtless, they are looking at going to CS5 before too long. I'd be running it on a T Series Thinkpad laptop - dual core 2.5GHz, 4GB DDR3, XP Pro 32 bit.
Worth doing? Wait for CS5? Or just stay with CS3? (I don't plan to upgrade my hardware anytime soon - two to three years off probably.)